Using Optical Ring Resonators for True-Time Delay: Investigating the Required Delay Bandwidth

Integrated microwave photonics is an emerging technology of which optical true-time delay networks are an ongoing research topic. Optical true-time delay is directly transferred to true-time delay in the microwave domain after optical heterodyning. This time delay is used to control the steering direction of a phased array antenna in wireless communication systems. Optical ring resonators are often considered for the creation of optical true-time delay networks. However, the literature is inconsistent about the required delay bandwidth. In this paper, it is concluded that the entire optical signal (both tones and modulation) needs to experience a flat group delay response in order to get the proper delay in the microwave domain. This implies that the required delay bandwidth is larger than what is often assumed. Simulations are presented that support this statement.